A few peptide residues in structurally important locations often determine biological functions of proteins implicated in the regulation of angiogenesis. We have shown recently that the short COOH-terminal segment PF-447–70 derived from platelet factor 4 (PF-4) is the smallest sequence that conserves potent antiangiogenic activity in vitro and in vivo. Here we show that modified COOH-terminal PF-4 peptides containing the sequence ELR (or related DLR), a critical domain present in proangiogenic chemokines, surprisingly elicit several times greater antiangiogenic potential than the original peptide. The modified peptides inhibit binding of iodinated vascular endothelial growth factor and fibroblast growth factor 2 to endothelial cell receptors, endothelial cell proliferation, migration, and microvessel assembly in the rat aortic ring model at lower doses than PF-447–70. On the differentiated chick chorioallantoic membrane, topical application of 40 μg of modified peptides potently reduces capillary angiogenesis induced by vascular endothelial growth factor165, a dose where peptide PF-447–70 was inactive. Established intracranial glioma in nude mice decreased significantly in size when treated locally with a total dose of 250 μg of peptide PF-447–70DLR (n = 10) compared with the same dose of the original PF-447–70 peptide (n = 10) or controls (n = 30). Tailored PF-4 peptides represent a new class of antiangiogenic agents with a defined mode of action and a strong in vivo activity.

VEGFs3 and FGFs are among the most important angiogenic factors. FGF-2 binds to tyrosine kinase receptors on ECs in a heparan sulfate-dependent manner (1). FGF-2 and its receptors play a critical role during normal development and maintenance of the embryonic vasculature (2, 3); monoclonal antibodies against FGF-2 inhibit tumor growth in nude mice through blocking of angiogenesis (4). Impairing FGFR-1 signaling by pyrimidine derivates or dominant-negative receptors leads to angiogenesis inhibition in vitro and in vivo(3, 5, 6). A peptide derived from PF-4 interacts with FGF-2 in a defined manner (7), and counteracts its biological activity in vitro and in vivo(8, 9). FGF-2 also induces production of VEGF via an autocrine feedback loop (10), and blocking antibodies against FGF-2 can counteract angiogenic effects of VEGF in vitro(11).

In growing tumors and the ischemic retina, hypoxia is the main regulator of VEGF expression through induction of transcription factor hypoxia-inducible factor 1α (12). Thus, five VEGF isoforms are generated by alternative splicing that exhibit different biochemical properties; variant VEGF165 represents the most abundant form and is one of the strongest angiogenic growth factors (13, 14). The biological response of VEGF is mediated by two EC-specific tyrosine kinase receptors, both essential for normal development and tumor angiogenesis (15, 16, 17).

Different strategies have been applied to block VEGF/VEGFR interactions; these include monoclonal antibodies (17, 18), tyrosine kinase inhibitors (19), inhibitory peptides (20), and soluble dominant-negative receptors (21).

PF-4 belongs to the CXC-chemokines, which, in general, are proangiogenic when the tripeptide ELR precedes the first CXC-domain, but are antiangiogenic when this motif is absent (22). Exceptions are growth-related protein β, which contains an ELR motif but inhibits angiogenesis in vitro and in vivo(23), and the ELR-negative stromal-derived factor 1, which shows proangiogenic effects in vitro and in vivo(24, 25). Administration of full-length tetrameric (ELR-negative) PF-4 restrains tumor growth and metastasis in mice models via inhibition of angiogenesis (26, 27). Survival of mice is prolonged by transducing established intracerebral glioma with an adenoviral vector encoding a secreted form of PF-4 (28). PF-4 exerts its effects most likely via interfering with FGF-2 and VEGF binding to receptors (29, 30). This activity is conserved in a COOH-terminal portion of the chemokine (9).

Domain swapping experiments between chemokines implicated in angiogenesis and hematopoiesis have revealed regions of importance for their biological functions. Replacing the NH2-terminal DLQ domain of PF-4 with the corresponding ELR motif of the angiogenic CXC-chemokines IL-8 or neutrophil-activating protein 2 confers CXCR-2-dependent neutrophil activation of these proteins to PF-4 (31, 32). On the other hand, mutation of ELR to DLQ or DLR in IL-8 greatly reduces its biological activity (33). Thus, the ELR motif plays a decisive role in defining biochemical behavior in this family of proteins.

For that reason we tried to reverse biological activities of PF-447–70, described recently as the smallest antiangiogenic portion of PF-4 (8) by inserting ELR or DLR mutations. The modified peptides surprisingly inhibited binding of iodinated VEGF or FGF-2 to cellular receptors at several times lower concentrations as the unmodified peptide and abrogated VEGF or FGF-2-induced EC proliferation but not glioma cell growth. Using a newly developed antiangiogenesis assay on the differentiated CAM, where capillary angiogenesis is induced in the stroma by VEGF, we show antiangiogenic effects of the modified peptides at a dose where the unmodified peptide was inactive. Enhanced inhibitory effects on tumor growth at lower doses than with the original peptide were observed in an intracranial glioma model. Taken together, our results show that a few amino acid residues within an antiangiogenic peptide can determine to a great extent the biological activity in vitro and in vivo. Modified PF-4 peptides represent good candidates for the development of peptide-based antiangiogenic drugs for cancers with high expression of VEGF and FGF-2.

Synthetic Peptides.

COOH-terminal PF-4 peptides (PF-447–70: NGRKICLDLQAPLYKKIIKKLLES; PF-447–70ELR: NGRKICLELRAPLYKKIIKKLLES; PF-447–70DLR: NGRKICL-DLRAPLYKKIIKKLLES; and PF-447–70S: NGRKISLDLQAPLYKKIIKKLLES) were purchased from ThermoHybaid (Ulm, Germany) or were donated by Rhône-Poulenc Rorer (Paris, France). All of the peptides were purified by high-performance liquid chromatography to >95%, and calculated molecular weight was confirmed by mass spectroscopy.

Growth Factors.

Recombinant human FGF-2 was kindly provided by Dr. Hervé Prats (Institut National de la Santé et de la Recherche Médicale U397, Toulouse, France) and stored in sterile, double-distilled water at −80°C. Recombinant human VEGF165 was produced in insect cells and purified as described elsewhere (34, 35). Human VEGF165-encoding baculovirus was a kind gift of Dr. Jean Plouët (Institut de Pharmacologie et de Biologie Stucturale, UMR 5089, Toulouse, France).

Cells.

BCE cells were a kind gift of Dr. Daniel B. Rifkin (New York University Medical Center, New York, NY). BAE cells were from Dr. Georg Breier (Department of Molecular Biology, Max-Planck-Institut fur physiologische und klinische Forschung, Bad Nauheim, Germany). ACE cells were donated by Dr. Jean-Jacques Feige (Institut National de la Santé et de la Recherche Médicale EPI 0105, CEA-Grenoble, France). All of the ECs were grown at 37°C, 5% CO2 in DMEM, 10% NBCS, and 1% glutamine and antibiotics (Life Technologies, Inc., Cergy Pontoise, France), and were used up to passage 25. BCE and ACE cells were grown in the presence of 2 ng/ml of FGF-2. C6 rat glioma cells (a kind gift from Dr. Paul Canioni, Centre National de la Recherche Scientifique UMR 5536, Université Bordeaux II, Bordeaux, France) were grown in DMEM, 10% FBS and antibiotics. U87 glioma cells (American Type Culture Collection) were grown in MEM α medium, 10% FBS, plus antibiotics. HI5 insect cells were a kind gift of Dr. Jean Plouët and were cultured without CO2 at 28°C in IPL41 insect cell medium (Life Technologies, Inc.) containing 10% FBS, l-glutamine, and antibiotics.

Binding Assays.

VEGF165 and FGF-2 were labeled with Na125I using Iodogen (Pierce, Rockford, IL) as a coupling agent according to the manufacture’s instructions (36). ACE cells were seeded at 2.5 × 105 density in gelatin-coated six-well plates and cultured in complete medium for 2 days. Cells were washed twice with ice-cold PBS, and incubated with 10 ng/ml 125I-labeled FGF-2 and peptides at indicated concentrations in binding medium [DMEM, 20 mm HEPES (pH 7.4), and 0.15% gelatin] for 2 h on a shaker at 4°C. 125I-labeled FGF-2 from high and low affinity binding sites was recovered and quantified as described earlier (8). 125I-labeled VEGF binding was evaluated essentially in the same manner, except that cells were detached (2% Triton, 10% glycerol, and 1 mg/ml BSA) before gamma counting. Each condition was tested in duplicates. Data are expressed as percentage of total radioactivity, and IC50 for every peptide was calculated from smooth curve fits.

Proliferation Assays.

ACE cells were seeded in six-well culture plates overnight in 10% NBCS at 5,000–7,500 cells/well. Medium was changed to 1% NBCS and 10 ng/ml FGF-2, and peptides at indicated concentrations were added to duplicate wells. FGF-2-stimulated controls were not treated. After 48 h, medium was changed, and stimulation with FGF-2 and peptide treatment repeated. One day later, cells were counted on a Coulter counter. For VEGF165-induced cell proliferation (10 ng/ml), BAE cells were used in conditions similar to ACE cells. IC50s were determined from smooth curve fits. U87 (C6) glioblastoma cells were seeded at 5,000 (25,000) cells/well in 10% FBS, treated with peptides at the same concentrations as ECs, and counted 72 h (48 h) later.

Migration Assay.

Migration tests with BAE or BCE cells were performed using a method described earlier (8, 37). In brief, ECs were seeded in 35-mm culture plates and were allowed to grow to confluence. Complete medium was replaced with serum-free DMEM, and incubation was continued overnight. One linear scar was drawn in the monolayer and divided into five equal fields. A set of digital photos was taken of each scar, and the denuded area was marked using digital image analysis software (Lucia G).4 The dishes were washed, and fresh serum-free medium containing 0.1% BSA, 10 ng/ml FGF-2 (BCE migration), or 10 ng/ml of VEGF165 (BAE migration) and peptides were added. Peptide concentration was 5 μm for BAE cells and 10 μm for BCE cells. After 18 h, cells were fixed in 1% glutaraldehyde, counterstained (Giemsa), and a second set of photos was taken. Photos were superposed, and ECs that migrated across the line drawn at the border of the scar in the first photo set were counted. Each condition was tested in duplicates in two independent experiments. Means for all fields of each group were calculated, background migration subtracted, and plotted as percentage of the mean of untreated stimulated control.

Rat Aortic Ring Model.

Cultures of rat aortic rings were prepared as described earlier (8). Briefly, aortic rings from male or female Sprague Dawley rats, between 200 and 500 grams, (IFFA CREDO, L’Arbresle, France or HARLAN, Gannat, France) were placed in 24-well dishes in collagen gel and serum-free MCDB131 medium. Rings were treated with peptides on days 0, 2, and 4, and photos were taken on day 7; controls received no peptide or peptide PF-447–70S. Microvessel length and number were measured using digital image analysis software (Lucia G).4 Statistical analysis of the differences in vessel length and vessel number between the treatment groups was performed by the ANOVA analyses of variance followed by Student’s Newman-Keul pair wise comparison (CRUNCH Software Corporation, Oakland, CA).

CAM Antiangiogenesis Assay.

Fertilized chicken eggs (Gallus gallus; E.A.R.L. Morizeau, Dangers, France) were incubated at 37°C and 80% humidified atmosphere. On day 4 of development, a window was made in the eggshell after punctuating the air chamber and sealed with Durapore tape. On day 13, plastic rings (made from Nunc Thermanox coverslips) were put on the CAM. Three μg of VEGF165 was premixed with 20 μg of peptides or with the equivalent volume of sterile water alone, and deposed in the center of the plastic ring. Treatment was repeated the following day. On day 17, the CAMs were fixed in vivo with 4% paraformaldehyde for 30 min at room temperature, and the area containing the ring was cut out for additional analysis. Photos of each CAM were taken under a stereomicroscope (Nikon SMZ800) using a digital camera (Nikon Coolpix 950). Two observers scored the inhibition of VEGF165-induced angiogenesis from 0 to 2 (0 = none, 1 = medium, and 2 = high). Means of the obtained scores were analyzed by the Kruskal-Wallis one-way ANOVA and pair wise comparisons of the different treatment groups (CRUNCH Software Corporation). For histological studies, samples were fixed in 3% glutaraldehyde and 2% formaldehyde in 0.12 m sodium cacodylate buffer, postfixed in 1% osmium solution, immersed with uranyl acetate, and embedded in Epon resin (Serva, Germany). Semithin sections (0.75 μm) were cut from samples using an Ultracut S microtom (Leica, Bensheim, Germany) and stained with 1% methylene blue and 1% azure II (Fluka, Buchs, Switzerland). Photos (×200 magnification) were taken using a Leica DMR microscope.

Intracranial Glioma Model.

Groups of 10 6-week-old nude mice (Charles Rivers Italia, Monza, Italy) were implanted intracranially with 50,000 human U87 glioma cells using an open window technique (38). After 15 days, animals were implanted with 2004 Alzet osmotic minipumps (ALZET, Cupertino, CA). The pump reservoir was connected to an intracranial catheter, placed slightly posterior to the tumor cell injection site, in the same hemisphere. The pump reservoir was filled with 0.25, 0.5, or 1 mg of the peptides in PBS. Control groups received pumps containing PBS or no pumps at all. Animals were sacrificed 30 days after pump implantation. Brains were removed, immediately frozen in liquid nitrogen, and embedded in OCT. Sections (5 μm) were made and processed for histology. Tumor volumes were measured from histology sections using the ellipsoid formula (width 2 × length/2; Ref. 38). Statistical analysis of tumor volumes was performed with a two-way ANOVA (peptide × dose) followed by analyses of simple main effects to compare the effects of the three peptides at each dose. Pairwise comparisons between peptides at each dose were performed by the Newman-Keuls post-test. Immunohistochemistry was carried out using the Vectastain Elite kit (Vector Laboratories, Burlingame, CA). Anti-CD31 antibody (PharMingen, San Diego, CA) was used at 1:100 dilution. Signal was visualized with 3,3′-diaminobenzidine chromogen, and sections were counterstained with hematoxylin. Microvessel counts and density were scored as reported previously (39). Apoptotic cells were detected with the ApopTag plus kit (Intergen, Purchase, NY) and quantified as described earlier (39). Animal studies (intracranial glioma model) were approved by the ethical committee of the University of Milano.

Modified PF-4-derived Peptides Inhibit Binding of VEGF and FGF-2 to Their Receptors.

First, peptides were tested for their ability to interfere with binding of 125I-labeled FGF-2 and 125I-labeled VEGF165 to capillary ECs, which express receptors for both families of growth factors (36, 40). A concentration-dependent inhibition of binding was observed with modified peptides. IC50 necessary to inhibit 125I-labeled VEGF165 binding to its receptors were 0.4 μm for PF-447–70DLR, 0.56 μm for PF-447–70ELR, and 1.58 μm for PF-447–70 (Fig. 1,A). IC50 for inhibition of FGF-2 binding to low or high (in brackets) affinity sites were 0.75 μm (0.46 μm) for PF-447–70DLR, 1.51 μm (0.53 μm) for PF-447–70ELR, and 3.47 μm (2.06 μm) for peptide PF-447–70 (Fig. 1, B and C). Control peptide PF-447–70S at the highest concentration tested in this assay did not compete for receptor binding.

Inhibition of EC but Not Glioma Cell Proliferation.

EC proliferation is a key step in the angiogenic process, and strongly inducible by VEGF165 (35) and FGF-2 (41). When BAE cells were stimulated by VEGF, PF-447–70DLR showed the strongest inhibitory activity (IC50 1.57 μm; Fig. 2,A), followed by PF-447–70 (2.15 μm) and PF-447–70ELR (2.91 μm). The differences were more pronounced for FGF-2-induced ACE cell proliferation: PF-447–70ELR and PF-447–70DLR exhibited IC50 values of 1 μm and 2.34 μm in comparison to 11.17 μm for PF-447–70 (Fig. 2,B). In contrast to ECs, proliferation of two glioma cell lines, human U87 and rat C6 cells, was not inhibited by normal or modified PF-4 peptides at all of the doses (Fig. 2 C).

Inhibition of Micro- and Macrovascular EC Migration.

Without stimulation, some random background migration occurred (Fig. 3,A). BAE cells migrated into the denuded scar area when stimulated with VEGF165 (Fig. 3,B) and, in a very similar way, when FGF-2 was added to BCE cells (data not shown). We tested the modified peptides at a concentration where PF-447–70 was inactive or much less active (5 μm on BAE cells and 10 μm on BCE cells). PF-447–70ELR strongly inhibited EC migration (Fig. 3, E and G); PF-447–70DLR induced complete inhibition of VEGF-induced migration (Fig. 3, F and G), PF-447–70S showed no effect (Fig. 3,C) and PF-447–70 showed only a small one (Fig. 3, D and G). One representative BAE migration experiment is shown; modified peptides showed a similar increase in activity compared with PF-447–70 in FGF-2-induced BCE migration (data not shown).

Blocking of Microvessel Assembly in the Rat Aortic Ring Model.

In the absence of peptides, aortic rings develop an extensive network of sprouting microvessels after 5–7 days (Fig. 4,A). PF-447–70S at the highest concentration tested in this assay did not show any effects on vessel length or number (Fig. 4, E and F). A strong reduction of the mean microvessel length was observed only for peptide PF-447–70DLR at 10 μm compared with the same dose of PF-447–70 (P < 0.05) or PF-447–70ELR [P < 0.002; Newman-Keul posthoc test after ANOVA analysis of all groups: F(7.79) = 11.23; P < 0.001; Fig. 4, D-F].

Statistical analysis of peptide effects on microvessel number showed that groups differed significantly from each other [one-way ANOVA: F(7.79) = 28.37; P < 0.001]. Inhibition (40%) was observed in cultures treated with 5 μm PF-447–70 and 63.5% at 10 μm compared with PF-447–70S. Mean vessel number was reduced by 33% at 5 μm (40% at 10 μm) in cultures treated with PF-447–70ELR compared with control peptide PF-447–70S. Fifty percent inhibition was reached at 5 μm, and 90% inhibition was achieved at 10 μm in cultures treated with PF-447–70DLR (Fig. 4 F). Thus, in this assay, only peptide PF-447–70DLR showed a clear increase in inhibitory activity in comparison to PF-447–70. Posthoc pairwise comparison between PF-447–70DLR and PF-447–70 or PF-447–70ELR proved the difference in activity to be significant [P < 0.001; Newman-Keul: F(7.79) = 11.23].

Inhibition of VEGF-induced Angiogenesis on the CAM.

Three μg of recombinant human VEGF165 applied in the center of a plastic ring on the surface of the CAM induced strong capillary formation (Fig. 5,B). The effect of VEGF is also present around the site of application because of diffusion. Water alone had no effect (Fig. 5,A). When premixed with the growth factor and deposed on the CAM, a clear antiangiogenic effect of peptide PF-447–70ELR and PF-447–70DLR was visible inside the ring (Fig. 5, C and D). PF-447–70ELR also causes a hyperplasia of the chorionic epithelium, comparable with hyperosmolar effects (Fig. 5 C, inset).

Control peptide PF-447–70S did not inhibit VEGF-induced capillary growth in the CAM stroma (Fig. 5,E). At the low dose tested, PF-447–70 failed to stop capillary growth in this assay (Fig. 5,F) and did not differ from the control peptide (P < 0.05; after analysis of differences between all treatment groups, Kruskal-Wallis test: H (4) = 28.27, P < 0.0001). The modified peptides PF-447–70ELR and PF-447–70DLR both showed an equally strong antiangiogenic effect at the dose tested compared with PF-447–70S (** = P < 0.002 for both peptides; Fig. 5,G). In semi-thin sections of control CAMs treated with sterile water, the capillary layer stays within the chorionic epithelium (Fig. 5,A, arrow, inset); larger conduit vessels are present in the stroma, filled with erythrocytes. VEGF165 induces a strong de novo growth of capillaries, which are present throughout the stroma (Fig. 5,B, arrows, inset). Control peptide PF-447–70S does not neutralize the capillary angiogenesis induced by VEGF165 (Fig. 5,E, inset). PF-447–70 at the low dose tested (2 × 20 μg) also does not inhibit capillary angiogenesis (Fig. 5,D, inset). PF-447–70ELR (Fig. 5,C, inset) as well as PF-447–70DLR (Fig. 5 D, inset) markedly decreases angiogenesis nearly to control levels.

Inhibition of Intracerebral Tumor Growth.

Animals treated with control peptide PF-447–70S, with PBS, or those without pumps developed large tumors (Fig. 6,A, top panel). Animals receiving different doses of peptide PF-447–70DLR or PF-447–70 showed a strong reduction of tumor volumes (Fig. 6,A, lower panel). Immunohistochemical analysis with CD31 antibody revealed that inhibition of tumor growth in animals treated with PF-447–70DLR or PF-447–70 is associated with a decreased vessel density (Fig. 6,B) and an increased number of apoptotic cells (Fig. 6 C). Statistical analysis of tumor volumes of animals treated with the equal dose of active peptides (PF-447–70 and PF-447–70DLR) revealed a significant difference in antitumor efficacy and showed a clear advantage of peptide PF-447–70DLR, especially at the lowest dose tested, 0.25 mg (**P < 0.005).

PF-447–70 is the smallest antiangiogenic fragment of PF-4 with strong activity in vitro and in vivo(8). The peptide contains a DLQ motif, essential for inhibition of myeloid progenitor proliferation by full-length PF-4 (33). When we replaced it with the ELR tripeptide, present in proangiogenic chemokines like IL-8 or neutrophil-activating protein 2, we expected a peptide with proangiogenic activity. In contrast, antiangiogenic activity was enhanced compared with PF-447–70 in a number of assays. An even stronger effect was observed with PF-447–70DLR (Q56 to R), which had initially been designed as a control for PF-447–70ELR, because the ELR to DLR mutation in the NH2 terminus of IL-8 greatly diminishes its function (33). FGF-2 and VEGF165 binding to tyrosine kinase receptors on ECs is strongly suppressed by PF-447–70ELR and −DLR. Endostatin also inhibits VEGF165 binding to VEGFR-2, but the effect seems to depend greatly on preincubation of endostatin with cells before addition of iodinated VEGF165(42). Disturbing VEGF and FGF-2 functions at the receptor level leads to impairment of fundamental angiogenic events such as EC proliferation and migration. Modified peptides inhibited both micro- and macrovascular EC proliferation and migration induced by VEGF165 or FGF-2 in the low micromolar range. Comparable results have been obtained with a designed peptide inhibitor termed “Anginex,” a 33-mer β-sheet-forming peptide containing sequences from PF-4, IL-8, and bactericidal-permeability increasing protein (43). However, Anginex doses necessary for inhibition of EC migration were several times higher than those of PF-447–70ELR or −DLR, and only FGF-2-induced angiogenesis has been studied. COOH-terminal PF-4 peptides inhibit EC proliferation below 1% serum controls from a certain concentration on, which suggests that residual VEGF and/or FGF-2 (or other mitogenic factors) present in serum were also inhibited. Peptide PF-447–70DLR was a strong inhibitor of vascular sprouting, but growth of fibroblastic cells was not affected, which suggests that the peptide antagonizes preferentially factors supporting EC growth and organization. PF-447–70ELR did not inhibit microvessel sprouting very well in this assay. Because the ELR motif is crucial for receptor activation of the proangiogenic chemokine IL-8 (44), it is possible that PF-447–70ELR stimulates chemokine receptors on residual leukocytes and ECs (45) present in the aortic wall, which may partially overcome the direct inhibitory effects on endogenous VEGF and FGF-2. Endostatin completely inhibits vascular sprouting in the aortic ring assay at 500 μg/ml (46), whereas PF-447–70DLR shows maximal activity between 14 and 28 μg/ml (5–10 μm), thus, at 18–35 times lower concentrations. This strong activity is probably because the peptide interferes with both, VEGF and FGF-2, by inhibition of binding to their receptors.

We tested effects of COOH-terminal PF-4 peptides for their ability to interfere with VEGF165-induced angiogenesis on the differentiated day 13 CAM (47), to our knowledge the first attempt to study angiogenesis inhibitors in this modified and rigorous type of assay. The typical brush-like formation of capillaries in the stroma of the CAM induced by human recombinant VEGF165 is strongly reduced by PF-447–70DLR and −ELR. In some eggs, PF-447–70 and PF-447–70S also showed some minor activity, but this may result from interindividual variations, the high sensitivity of the assay system, or the difficulty to exactly quantify differences between low inhibitory effects. Metastatin, a recently described angiogenesis inhibitor, was active in the day 10 CAM assay at a two times higher dosage. Additionally, angiogenesis was induced with VEGF at a 300-fold lower dose as in our assay (48). Cyclic peptide antagonists for αVβ3-integrin showed strong antiangiogenic effects in the day 10 CAM at 300 μg; again, the angiogenic response was induced by VEGF or FGF-2 at doses <1 μg (49). These comparisons indicate that modified COOH-terminal PF-4 peptides inhibit VEGF-induced angiogenesis in vivo in a very efficient way. Both peptides start with the NGR tripeptide, a motif, which has been shown recently to be a homing sequence to angiogenic blood vessels (50). This might also contribute to their strong in vivo activity. Glioma growth is strongly promoted via up-regulation of angiogenesis by VEGF and FGF-2. U87 glioma cells produce high levels of these growth factors in culture (51). Two-hundred-fifty μg of peptide PF-447–70DLR were already sufficient to reduce tumor growth to one-half compared with PF-447–70-treated animals. This effect is paralleled by an increased reduction of blood vessels within the tumor, and elevated tumor and/or EC cell apoptosis. Increased apoptosis in tumors is commonly observed after treatment with angiogenesis inhibitors (52, 53, 54). Similar results have been obtained previously by systemic treatment of U87 glioma-bearing mice with the antiangiogenic molecule PEX alone or in combination with low-dose chemotherapy. When PEX was included in the treatment, animals survived significantly longer and showed no measurable side effects (38, 39).

Binding of FGF-2 or VEGF165 to tyrosine kinase receptors is facilitated by heparan sulfate proteoglycans on the cell surface and is essential for their proangiogenic activities (55). It is possible that the peptides, which contain the COOH-terminal heparin-binding motif of PF-4 consisting of two double-lysine clusters (56), operate in part via inhibition of this interaction. Three positively charged arginines within full-length PF-4 (R20, R22, and R49) contribute to its particularly high heparin-binding capacity (57). This might explain the improved antiangiogenic activity, because both PF-447–70ELR and −DLR contain an additional arginine (R56) together with the internal R49. Similar observations have been made with peptides mimicking the surface of endostatin: an increase of arginine residues enhances their antiangiogenic effects probably because of an increased affinity for heparin (58).

It has been shown recently that unmodified PF-447–70 associates directly with FGF-2, independently of glycosaminoglycan binding (7), leading to a conformational change of the growth factor. It is possible that the increase in activity observed with the modified peptides is also because of an increase in affinity for FGF-2 and perhaps VEGF too. Taken together, modified COOH-terminal peptides of PF-4 are potent inhibitors of VEGF- and FGF-2-induced angiogenesis in vitro and in vivo. They might be especially useful in the treatment of tumors expressing high levels of these growth factors, like glioblastoma.

Fig. 1.

ACE cells were incubated with 10 ng/ml of 125I-labeled VEGF165 (A) or 125I-labeled FGF-2 (B and C) and peptides at indicated concentrations. Control peptide PF-447–70S at the highest dose tested (20 μm) had no effect. IC50s were calculated from smooth curve fits. Peptide PF-447–70ELR inhibited 125I-labeled VEGF binding to VEGFRs at approximately three times lower concentrations and PF-447–70DLR at four times lower concentrations than peptide PF-447–70 (A). FGF-2 binding to low affinity receptors (LA FGFR) was affected in a similar way: PF-447–70ELR-induced inhibition of binding was two times stronger and PF-447–70DLR-induced inhibition nearly five times stronger than that of PF-447–70 (B). FGF-2 binding to high affinity receptors (HA FGFR) was inhibited at approximately four times lower concentrations by peptides PF-447–70ELR and PF-447–70DLR than by peptide PF-447–70 (C).

Fig. 1.

ACE cells were incubated with 10 ng/ml of 125I-labeled VEGF165 (A) or 125I-labeled FGF-2 (B and C) and peptides at indicated concentrations. Control peptide PF-447–70S at the highest dose tested (20 μm) had no effect. IC50s were calculated from smooth curve fits. Peptide PF-447–70ELR inhibited 125I-labeled VEGF binding to VEGFRs at approximately three times lower concentrations and PF-447–70DLR at four times lower concentrations than peptide PF-447–70 (A). FGF-2 binding to low affinity receptors (LA FGFR) was affected in a similar way: PF-447–70ELR-induced inhibition of binding was two times stronger and PF-447–70DLR-induced inhibition nearly five times stronger than that of PF-447–70 (B). FGF-2 binding to high affinity receptors (HA FGFR) was inhibited at approximately four times lower concentrations by peptides PF-447–70ELR and PF-447–70DLR than by peptide PF-447–70 (C).

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Fig. 2.

Proliferation was stimulated with VEGF165 using BAE cells (A) or FGF-2 and ACE cells (B). PF-447–70, PF-447–70ELR, and PF-447–70DLR inhibited cell proliferation in a concentration-dependent manner; peptide PF-447–70S did not show any effects at all concentrations tested. Horizontal line at 0 indicates conditions where 100% inhibition of VEGF- or FGF-2-induced proliferation is reached. IC50s were determined from smooth curve fits and were in the micromolar range, with the following order of activity (highest to lowest): PF-447–70DLR, PF-447–70, PF-447–70ELR for VEGF-induced proliferation and PF-447–70DLR, PF-447–70ELR, PF-447–70 for FGF-2-induced proliferation. In contrast, serum-induced U87 or C6 glioma cell proliferation was not affected by the peptides (C).

Fig. 2.

Proliferation was stimulated with VEGF165 using BAE cells (A) or FGF-2 and ACE cells (B). PF-447–70, PF-447–70ELR, and PF-447–70DLR inhibited cell proliferation in a concentration-dependent manner; peptide PF-447–70S did not show any effects at all concentrations tested. Horizontal line at 0 indicates conditions where 100% inhibition of VEGF- or FGF-2-induced proliferation is reached. IC50s were determined from smooth curve fits and were in the micromolar range, with the following order of activity (highest to lowest): PF-447–70DLR, PF-447–70, PF-447–70ELR for VEGF-induced proliferation and PF-447–70DLR, PF-447–70ELR, PF-447–70 for FGF-2-induced proliferation. In contrast, serum-induced U87 or C6 glioma cell proliferation was not affected by the peptides (C).

Close modal
Fig. 3.

EC migration was stimulated with 10 ng/ml VEGF165 on BAE cells. During the 18-h period in serum-free medium, some random cell migration occurs (A). VEGF165 induces cell migration into the denuded area of the scar (B). At 5 μm, control peptide PF-447–70S (C and G) had no effects. PF-447–70 (D and G) only slightly antagonizes promigratory effects of VEGF165. PF-447–70ELR (E and G) had profound effects on VEGF165-induced migration; PF-447–70DLR (F and G) suppressed it below control levels.

Fig. 3.

EC migration was stimulated with 10 ng/ml VEGF165 on BAE cells. During the 18-h period in serum-free medium, some random cell migration occurs (A). VEGF165 induces cell migration into the denuded area of the scar (B). At 5 μm, control peptide PF-447–70S (C and G) had no effects. PF-447–70 (D and G) only slightly antagonizes promigratory effects of VEGF165. PF-447–70ELR (E and G) had profound effects on VEGF165-induced migration; PF-447–70DLR (F and G) suppressed it below control levels.

Close modal
Fig. 4.

Rat aortic rings were incubated in MCDB131 medium without serum and growth factors (A), treated with PF-4-derived peptides at 10 μm (B–D), or at indicated doses (E and F). Peptide PF-447–70S did not interfere with microvessel growth, whereas peptides PF-447–70 (B), and its derivates PF-447–70ELR (C) and PF-447–70DLR (D) impaired this process; with different efficacy at the same dose (E and F). Mean vessel length (E) is strongly reduced under treatment with 10 μm PF-447–70DLR compared with the same dose of PF-447–70 (P < 0.05) or PF-447–70ELR (P < 0.002). When comparing mean vessel number, PF-447–70 showed a dose-dependent inhibition, whereas PF-447–70ELR displayed only a moderate, not dose-dependent effect. PF-447–70DLR strongly reduced mean vessel number in a dose-dependent manner (PF-447–70DLR versus PF-447–70 or PF-447–70ELR; P < 0.001). Data for statistical analysis were pooled from three independent experiments and are plotted as mean; bars, ± SE. Photos (×40 magnification) were taken from one representative experiment.

Fig. 4.

Rat aortic rings were incubated in MCDB131 medium without serum and growth factors (A), treated with PF-4-derived peptides at 10 μm (B–D), or at indicated doses (E and F). Peptide PF-447–70S did not interfere with microvessel growth, whereas peptides PF-447–70 (B), and its derivates PF-447–70ELR (C) and PF-447–70DLR (D) impaired this process; with different efficacy at the same dose (E and F). Mean vessel length (E) is strongly reduced under treatment with 10 μm PF-447–70DLR compared with the same dose of PF-447–70 (P < 0.05) or PF-447–70ELR (P < 0.002). When comparing mean vessel number, PF-447–70 showed a dose-dependent inhibition, whereas PF-447–70ELR displayed only a moderate, not dose-dependent effect. PF-447–70DLR strongly reduced mean vessel number in a dose-dependent manner (PF-447–70DLR versus PF-447–70 or PF-447–70ELR; P < 0.001). Data for statistical analysis were pooled from three independent experiments and are plotted as mean; bars, ± SE. Photos (×40 magnification) were taken from one representative experiment.

Close modal
Fig. 5.

Sterile water and plastic rings alone did not interfere with CAM vasculature (n = 8; A). A mixture of VEGF165 and water (n = 17; B) or VEGF165 and 20 μg of peptides PF-447–70S (n = 13; E), PF-447–70 (n = 12; F), PF-447–70ELR (n = 14; C), and PF-447–70DLR (n = 13; D) was placed in the center of the rings. VEGF165 induces brush-like capillary formation surrounding precapillary arterioles (B). Control peptide PF-447–70S (E) and peptide PF-447–70 (F) were not able to counterbalance growth of new capillaries. Both modified peptides PF-447–70ELR (C) and DLR (D) had a strong antiangiogenic effect in the CAM; statistical analysis proved the difference compared with PF-447–70S to be significant (∗∗P < 0.002; G). Semithin sections of controls show that the capillary layer stays within the chorionic epithelium (arrow, inset in A); larger conduit vessels are present in the stroma, filled with erythrocytes. VEGF165 induces a strong de novo growth of capillaries, which are present throughout the stroma (arrows, inset in B). Control peptide PF-447–70S does not neutralize the capillary angiogenesis induced by VEGF165 (inset in E). PF-447–70 at the low dose tested (2 × 20 μg) also does not inhibit capillary angiogenesis (inset in F). PF-447–70ELR and DLR both markedly decreases angiogenesis nearly to control levels (insets in C and D).

Fig. 5.

Sterile water and plastic rings alone did not interfere with CAM vasculature (n = 8; A). A mixture of VEGF165 and water (n = 17; B) or VEGF165 and 20 μg of peptides PF-447–70S (n = 13; E), PF-447–70 (n = 12; F), PF-447–70ELR (n = 14; C), and PF-447–70DLR (n = 13; D) was placed in the center of the rings. VEGF165 induces brush-like capillary formation surrounding precapillary arterioles (B). Control peptide PF-447–70S (E) and peptide PF-447–70 (F) were not able to counterbalance growth of new capillaries. Both modified peptides PF-447–70ELR (C) and DLR (D) had a strong antiangiogenic effect in the CAM; statistical analysis proved the difference compared with PF-447–70S to be significant (∗∗P < 0.002; G). Semithin sections of controls show that the capillary layer stays within the chorionic epithelium (arrow, inset in A); larger conduit vessels are present in the stroma, filled with erythrocytes. VEGF165 induces a strong de novo growth of capillaries, which are present throughout the stroma (arrows, inset in B). Control peptide PF-447–70S does not neutralize the capillary angiogenesis induced by VEGF165 (inset in E). PF-447–70 at the low dose tested (2 × 20 μg) also does not inhibit capillary angiogenesis (inset in F). PF-447–70ELR and DLR both markedly decreases angiogenesis nearly to control levels (insets in C and D).

Close modal
Fig. 6.

Locally administered peptide PF-447–70 or PF-447–70DLR strongly reduced tumor volumes of established intracerebral tumors in nude mice at all three doses (A, bottom panel). PF-447–70DLR displayed a greater antitumor activity than PF-447–70 at all doses. This difference was more pronounced at the lowest dose (0.25 mg) than at the two higher doses (two-way ANOVA followed by the Newman-Keuls post-test: 0.25 mg: ∗∗P < 0.005; 0.5 mg: ∗P < 0.02; 1 mg: ∗P < 0.03). PBS-treated animals or animals without pump (controls) had several times greater tumor volumes, and the two groups did not differ significantly from each other (Student’s t test; t = 30, degrees of freedom = 18; NS; A, top panel). Control peptide PF-447–70S did not have any influence on tumor growth and volumes differed significantly from peptide-treated animals at all doses (P < 0.0001). Reduced microvessel numbers and elevated apoptosis indices suggest inhibition of angiogenesis as the pathophysiological mechanism of action in animals treated with PF-447–70 or PF-447–70DLR (B and C).

Fig. 6.

Locally administered peptide PF-447–70 or PF-447–70DLR strongly reduced tumor volumes of established intracerebral tumors in nude mice at all three doses (A, bottom panel). PF-447–70DLR displayed a greater antitumor activity than PF-447–70 at all doses. This difference was more pronounced at the lowest dose (0.25 mg) than at the two higher doses (two-way ANOVA followed by the Newman-Keuls post-test: 0.25 mg: ∗∗P < 0.005; 0.5 mg: ∗P < 0.02; 1 mg: ∗P < 0.03). PBS-treated animals or animals without pump (controls) had several times greater tumor volumes, and the two groups did not differ significantly from each other (Student’s t test; t = 30, degrees of freedom = 18; NS; A, top panel). Control peptide PF-447–70S did not have any influence on tumor growth and volumes differed significantly from peptide-treated animals at all doses (P < 0.0001). Reduced microvessel numbers and elevated apoptosis indices suggest inhibition of angiogenesis as the pathophysiological mechanism of action in animals treated with PF-447–70 or PF-447–70DLR (B and C).

Close modal

The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

1

Supported by grants from the Ligue Nationale Contre le Cancer, the Institut National de la Santé et de la Recherche Médicale, the Ministère de la Recherche (all to A. B.); by the Ricerca Finalizzata, Ospedale Maggiore di Milano and Fondazione Monzino, Milan, Italy and the Associazione Italiana Ricerca Sul Cancro (AIRC) (to L. B.). M. H. was a fellowship of the Féderation de la Recherche Médicale (FRM) and the Féderation des Aveugles et Handicapés visuels de France (FAF). L. Z. was supported by the Association pour la recherche contre le cancer (ARC).

3

The abbreviations used are: VEGF, vascular endothelial growth factor; VEGFR, vascular endothelial growth factor receptor; FGF, fibroblast growth factor; FGFR, fibroblast growth factor receptor; PF-4, platelet factor 4; CAM, chorio-allantoic membrane; BCE, bovine capillary endothelial; NBCS, newborn calf serum; BAE, bovine aortic endothelial; ACE, adrenal cortex capillary endothelial; EC, endothelial cell; FBS, fetal bovine serum; IL, interleukin.

4

Internet address: http://www.lim.cz.

We thank S. Antoni and G. Frank (Anatomisches Insitut I, Universität Freiburg, Freiburg, Germany) for their excellent technical assistance, and Dr. Françoise Dellu-Hagedorn (UMR 5541, Neuropsychobiologie des désadaptations, Bordeaux, France) for help with the statistical analysis.

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